Add support for GitHub's new Light High Contrast theme

[kamekku14]

Echoing from #166, #176 and #178, there's now a new Light High Contrast theme for GitHub—and it's exactly the same as GitHub's Light theme, but with better contrast. This feature is enabled by default in user menu > Feature Settings for those who would like to test.

I would like to see if this browser extension fits very well for this theme.